1. Field of the Invention
The invention relates to an ultrasonic probe for injecting ultrasonic pulses into a body under investigation, for detecting resulting echo signals from within the body and for generating echo signal waveforms representing the echo signals detected.
2. Description of the Prior Art
Extensive research into real time ultrasonic imaging systems for use in medical diagnosis in the field of obstetrics, opthalmology and internal medicine has led to the development of a variety of devices performing a variety of imaging techniques. One form of such apparatus utilises an ultrasonic probe, incorporating an array of ultrasonic transducers, which in use is placed in contact with a patient's body and selected transducers energised to transmit ultrasound into the body. The ultrasound is reflected from acoustic impedance discontinuities within the body and the echoes are detected and converted by the transducers to electric echo signals. The time delays between the transmission of each ultrasonic pulse into the body and receipt of the echoes of that pulse are proportional to the distances from the various transducers to the discontinuities within the body producing the echoes. In practice, a plurality of transducers are used in a controlled manner to transmit and receive echoes of ultrasound and the resulting electric echo signals processed to construct a cross-sectional image of the body which is displayed on a cathode ray tube.
The number and size of the transducer elements in the array are determined having regard to the design requirements needed for the specific application. The pattern of energisation is selected to provide a beam of required characteristics for the application. Thus for example a linear array of transducers may be used, the angle of incidence of the beam being determined by the phase relationship between the energised elements in the array. In addition to beam steering, the intensity of sound may be enhanced in a selected direction from the centre of a linear array of transducers by phasing the transmission times from individual transmitting transducers. The received echoes are subsequently electronically focussed by reverse phasing of the received signals. This method suffers inevitably from random echoes caused by `sidelobes` of the transmitted soundwaves.
As an alternative to beam steering, the transducers in a linear array may be energised simultaneously, a group at a time, to achieve homogeneity of the wave and the received echoes routed through selected delay lines, thus achieving a measure of electronic focussing. This method suffers from echo artefacts due to the large area being "insonified" (i.e., bombarded with sound waves), and by the fact that delay lines provide only one part of the time transformation necessary for exact focussing.